A punctuation question with the same words in every choice tests where a sentence can be divided. Read the words on both sides of the blank and test whether they can stand alone. A period needs a full sentence on each side, and a colon needs one before it. Then name the parts on each side: no mark separates a verb from its object, the thing it acts on. A pause in your reading isn't a reason to add punctuation.
Hints
- Hint 1
Look at the words after the blank. Can they stand alone as a complete sentence?
- Hint 2
Ask what the birdwatchers observe. How do the words that answer that question connect to the verb?
Step-by-step
Test the sentence, then name its parts
Step 1Test the period and colon
Read the words after the blank, starting with annual migration of sandhill cranes. Can those words stand alone? It names and describes an event but has no main verb, so the period leaves a fragment, a group of words that can't stand alone. The colon also fails: it needs a full sentence before it, but observe still needs to say what the birdwatchers observe here.
- Step 2
Find the verb's object
What do the birdwatchers observe? The words annual migration of sandhill cranes across the Platte River basin answer that question, so they're the object of observe, the thing the verb acts on. The later phrase about the spectacular event describes that same migration.
- Step 3
Keep the verb with its object
No punctuation goes between a verb and its object. Shorten the object to the migration: Birdwatchers observe the migration. The comma would split words that belong together, so we need observe the with no mark. Choice B.